In the development of synthetic filaments for the replacement of hog bristles in hair brushes, a wide variety of polymers has been explored. While oriented nylon monofilaments have been used most frequently in this application, effort has been directed toward the use of various polyesters in brush bristles to take advantage of the properties of these materials. An important characteristic for brush bristles is the ability of the bristles to retain their original shape with extended use. Typically, this retention of shape is designated as Mandrel Bend Recovery (MBR), that is, the extent to which a monofilament will recover its original configuration after being bent over a mandrel. In addition, of course, a brush bristle must exhibit satisfactory tensile strength for normal usage and good dimensional stability when exposed to elevated temperatures. Continuing effort has been directed toward the ideal balance of these properties for polyester filaments.